All facilities which burn sulfur-containing fossil fuel to generate electric power, as well as sulfide roasting industries and many other industrial processes, emit sulfur-dioxide and smaller amounts of sulfur-trioxide in their waste gas streams. Although the concentration of sulfur-oxides in the waste gas is usually low, the total amount emitted annually is excessive. The emissions present a severe pollution problem and constitute a loss of a valuable natural resource.
The U.S. Environmental Protection Agency, under authority of the Clean Air Act, as amended, has issued standards which call for a substantial reduction in the amount of sulfur discharged to the atmosphere. Compliance with these emission standards may be attained by use of low-sulfur fuel, fuel desulfurization, and gasification processes to make a clean fuel; however, sufficient supplies of such fuels are not now available and may not be so for some time to come. The alternative is to remove the sulfur-dioxide from the waste gas streams.
Many processes for the removal alone and removal with recovery of sulfur-oxides from waste gas streams have been proposed (Sulfur-Oxide Removal from Power Plants Stack Gas, McGlamery, G. G. et al, Environmental Protection Technology Series, EPA report EPA-R2-73-244, May, 1973; Status of Stack Gas Technology for SO.sub.2 Control EPRI report 209, part II, 1975; SO.sub.2 Abatement For Stationary Sources in Japan ANDO, J. and ISSACS, G. A., Environmental Protection Technology Series, EPS report EPS-600/2-76-013a, January, 1976; Flue Gas Desulfurization:An Overview, Slack, A. V., Chem. Eng. Progress 72,94-97, 1976). Absorbents such as slurries of metal oxides or hydroxides, aqueous solutions of ammonium and sodium salts, molten alkali salts, and solid absorbents such as sodiumaluminates and activated carbon have been evaluated. Of the many processes tested, most were effective in removing sulfur oxides from the waste gas streams but suffered from other defects such as poor economics, difficulty in regeneration of the absorbents, production of large quantities of a sulfur bearing waste product which posed difficult disposal problems, and process conditions which were too strigent to maintain in a commercial installation. Because of these difficulties, at present no process for the removal of sulfur-oxides from waste gas has been accepted on a wide scale by the electrical power industry or other commercial facilities.
It is convenient to divide desulfurization processes into two categories: throw-away processes and regenerative processes. In the former group, the sulfur is recovered from the stack gas in a form not amenable to recovery in a usable form. As the name implies, sulfur is recovered in the so-called regenerative processes in a salable form usually as sulfur, sulfur-dioxide or sulfuric acid.
Because throw-away processes require an inexpensive scrubbing agent, they are all based on calcidic limestone or lime. In limestone scrubbing, stack gas is washed with a recirculating slurry of calcite and reacted calcium salts. For highest removal efficiencies, a two-stage scrubber system, consisting of a Venturi and mobile bed scrubber is used to remove both particulate matter and gaseous sulfur-oxides. In this process, sulfur dioxide dissolves in water to yield a mixture of sulfite, bisulfite and hydronium ions. Limestone simultaneously dissolves in the scrubbing liquor and calcium and sulfite ions subsequently react to yield solid calcium, sulfite hemihydrate. Part of the sulfite ions also oxidizes to ultimately yield gypsum.
To gain additional sulfur-dioxide removal efficiency, lime may be substituted for limestone as a scrubbing agent. In making this substitution, the reaction velocity is increased but the final throw-away products are still calcium sulfite hemihydrate and gypsum.
In spite of the apparent simplicity of such systems, there are substantial operating difficulties. The circulating limestone slurry is erosive, particularly at high circulation rates. This potential for erosion necessitates the use of rubber lining in central areas of the scrubbing system, thus increasing capital costs. Solid deposits on process equipment is another major deterrent to successful operation of limestone or lime slurry processes. Much research and development has been lavished on this problem, but it can be eliminated only by meticulous attention to operating details (Borg Wardt, R.H., EPA/RTP pilot studies related to unsaturated operation of lime and limestone scrubbers, p.225, EPA/650/2-74-126a, 1974).
Another non-regenerative or throw-away process is the double-alkali process. In this process, sodium sulfite is substituted for lime or limestone as the scrubbing agent. During the scrubbing, sulfur-dioxide converts sodium sulfite to sodium bisulfite and the spent sulfite liquor is regenerated by contacting it with lime.
Similar reactions are possible with limestone. The insoluble calcium sulfite along with some calcium sulfate formed by oxidation is separated from the solution by settling and decantation. Sodium lost in this operation is replaced with sodium carbonate. The double alkali process is similar to lime or limestone scrubbing processes in that the sulfur-dioxide and a calcium base are converted to calcium sulfite and calcium sulfate.
By separating the scrubbing and regeneration steps, calcium utilization is increased and scaling problems are greatly reduced. These advantages are achieved at the expense of replenishing losses in the scrubbing liquor with relatively expensive sodium carbonate.
In the non-regenerative or throw-away processes, the final throw-away product is a thixotropic sludge comprised of fly ash, unreacted limestone or lime, calcium sulfite hemihydrate, gypsum and unreacted dolomite.
Partly because of the waste disposal problems associated with throw-away processes, numerous regenerative methods of flue gas desulfurization have been developed. Ammonium scrubbing, magnesia scrubbing, sodium scrubbing (Wellman-Lord process), and citrate scrubbing are among some of the more important of the many proposed desulfurization processes.
Ammonium scrubbing is a process which showed great promise and was extensively tested by the Tennessee Valley Authority for several years but was finally abandoned because an environmentally and esthetically objectionable plume formed when the scrubber-off gas contacted the atmosphere. No economically feasible method of eliminating the plume was developed and the process has fallen into disfavor.
Magnesia scrubbing with sulfur recovery is a stack gas scrubbing method under which also has been given considerable study. At least three major technological routes have been followed. American, Japanese, and Russian workers concentrated on the use of magnesium sulfite-magnesium oxide slurries. The Grillo Werks A.G. adds MnO.sub.2 to the magnesium slurry to increase sulfur dioxide absorption efficiency. Some paper mills use an acidic clear liquor of magnesium sulfite and bisulfite to simultaneously remove particulate matter and absorb sodium sulfur-dioxide in a single scrubber. Of these variations, the basic MgO--MgSO.sub.3 slurry process is the most advanced.
It uses two scrubbers in series; the first scrubber uses water to remove particulate matter and sulfur-trioxide and the sulfur-dioxide is removed in the second scrubber. Magnesium sulfite and magnesium sulfate are precipitated and recovered and these crystals are calcined between 800.degree. and 1100.degree. C. in the presence of coke or a reducing atmosphere to regenerate MgO and to expel SO.sub.2. A major disadvantage of this process is the high temperature calcination of the magnesium sulfite and magnesium sulfate. This step is energy intensive and will become more costly as the cost of fuel increases.
Sodium scrubbing with sulfur recovering (Wellman-Lord) is another process under extensive study. As in the double alkali process, a sodium sulfite solution scrubs sulfur-dioxide from the flue gas. The spent bisulfite-rich scrubbing liquor is decomposed by steam stripping to regenerate sodium sulfite and to expel sulfur-dioxide. The sulfur-dioxide is recovered as product and the sodium-sulfite is returned to the process. Oxidation of the sulfur-dioxide and formation of sodium-sulfate occurs in the process as would be expected. To control the sodium sulfate level in the scrubbing solution a side stream is removed and sent to a purge treatment section where the sodium sulfate is crystallized and removed. The mother liquor is returned to the process and the sodium removed as sodium sulfate is replaced with sodium carbonate.
Oxidation is the greatest problem associated with the process because it leads to the consumption of sodium carbonate to produce sodium sulfate which has relatively little value. This oxidation may be partially suppressed with inhibitors such as para-phenylenediamine but the cost of the inhibitors adds appreciably to the operating costs.
The citric acid process, developed by the U.S. Bureau of Mines, uses a mixture of citric acid, sodium citrate, and sodium bisulfate to scrub sulfur dioxide from particulate-free gas streams.
The process comprises the following steps:
1. Particulate matter and sulfuric acid mist are removed from a cooled gas stream (45.degree. to 65.degree. C.).
2. A sodium citrate, citric acid and sodium thiosulfate mixture scrubs SO.sub.2 from the cooled gas stream.
3. The spent scrubbing solution is regenerated by a reaction with hydrogen sulfide at 65.degree. C. to yield elemental sulfur.
4. Sodium sulfate, an oxidation product, is crystallized from a slipstream by cooling.
5. The sulfur product is removed from the regenerated scrubbing solution by oil flotation and fusion.
6. Hydrogen sulfide is manufactured by reacting 2/3 of the recovered sulfur with steam and natural gas.
Two problems immediately stand out in this otherwise interesting process. The disposal of sodium sulfate may pose problems and, more seriously, the use of natural gas for hydrogen sulfide production is unattractive.
In my prior U.S. Pat. No. 4,189,309, a system and method of desulfurization of flue gas is disclosed wherein the flue gas is first scrubbed with water and is then cooled to a temperature in the range 40.degree.-125.degree. F. in a secondary cooler. This secondary cooler is of the indirect heat exchange type, requires a substantial quantity of cooling water flow, and represents approximately 25% of the total cost of the system. Thus, it would be desirable to eliminate or at least reduce the size and cost of the secondary cooling stage if possible.
The difficulty, however, is that the secondary cooling stage is necessary to reduce the temperature of the flue gas to a sufficiently low value so that the water in the next following stage, the absorber, may absorb a significant fraction of the SO.sub.2 from the cooled flue gas and thereby maintain the size of the closed absorber/desorber system within reasonable limits. Moreover, my prior system is a balanced system in that the temperature T.sub.3 to which the gas is cooled by the secondary cooler, the temperature T.sub.4 of the water leaving the absorber and the temperature T.sub.5 of the water leaving the desorber (evaporator) are closely interdependent as is illustrated in FIG. 4 of my prior patent.
A system of the type discribed above is to be distinguished from historically older systems (see U.K. patent specification 1427 of 1883 to Ramsey) which employ alkali or alkali-based aqueous absorbing liquors to absorb SO.sub.2 with subsequent heating to effect recovery of the SO.sub.2. These systems, in general, effect absorption of SO.sub.2 at relatively high temperatures and consequently require liquor additives which will significantly reduce the vapor pressure of SO.sub.2 over the liquor solution at these temperatures. Thus, citing the aforesaid Ramsey U.K. patent, H. F. Johnstone reported data concerning equilibrium partial vapor pressures over solutions of ammonia-sulfur dioxide-water systems In Recovery of Sulfur-Dioxide from Waste Gases, Industrial and Engineering Chemistry (1935) Vol. 27, pages 587-593.
In a later article having the same title [Industrial and Engineering Chemistry (1938), Vol. 30, pages 101-109], Johnstone, H. J. Read and H. C. Blankmeyer reported additional data concerning partial vapor pressures over other solutions.
The Clark U.S. Pat. No. 1,908,731 of May 16, 1933 is a similar type of system using an aqueous solution of alkali hydroxides or alkali sulfites with the addition of a salt in weak base and a strong acid, e.g., aluminum chloride which increases the recovery of SO.sub.2 on regeneration at elevated temperatures.
The Bottoms U.S. Pat. No. 1,834,016 of Dec. 1, 1931 is a similar system using a certain organic ammonium compounds as the absorbing liquid and similar system are disclosed in the Bottoms U.S. Pat. No. 1,783,901 of Dec. 2, 1930 and its reissue U.S. Pat. No. Re. 18,958 of Sep. 26, 1933.
It is noteworthy that all of these systems, especially in those where an aqueous absorbing liquor is used, the liquor is a strong solution of the additive and the regeneration step always involves supplying heat with or without pressure reduction to regenerate the absorbing liquor.